1. Field of the Invention
The present invention relates to side bearings used in railroad cars and, more particularly, to a long-travel side bearing for use with articulated railroad cars to permit a controlled torsional flexibility across the articulation joint connecting the car body units.
2. Description of the Prior Art
A variety of constant contact side bearings have been developed for use in railroad cars. On a standard railroad car, the side bearing units are typically located directly on the rail truck bolster. On an articulated car, the side bearing units are typically located on a bracket mounted on the truck bolster.
Such a standard constant contact side bearing unit is commonly used in an attempt to control hunting of the railroad car. Hunting is a phenomenon caused by yaw axis rotation of the railroad car truck. As the cone-shaped wheels ride along the track, a yaw axis motion is induced in the truck. As the truck yaws, an upper portion of the side bearing is made to slide across the underside of the car body. The resulting friction produces an opposing torque which acts to prevent yaw motion. A further purpose of the standard side bearing is to limit the total roll motion of the car body.
In order to control or limit the roll axis rotation of the railroad car, the side bearings of the prior art are designed to provide a roll stop or motion limitation action. Some prior art side bearings act simply as roll limitation pads or stops which provide a simple roll prevention or limitation device. These side bearings do not necessarily continuously contact the car body and do not attempt to restrain yaw axis movement of the truck.
Other prior art side bearings typically consist of a housing containing spring loaded steel elements or elastomeric materials. These bearings bear continuously against the car body throughout the compression of the spring. The spring or elastomeric material acts essentially to keep the upper element in contact with the car body to prevent or restrict yaw axis motion of the truck. The maximum travel of such a standard side bearing is specified by the appropriate Association of American Railroads (A.A.R.) standard (Standard M-948-77) to be limited to 5/8 inch.
Each of these prior art devices is therefore directed to the restraint of yaw axis motion of the truck and limitation of roll axis movement of the car body units. For example, U.S. Pat. No. 4,712,487 discloses a standard side bearing with a housing containing an elastomeric device. The device is made of polymer material and has a non-linear compressive spring rate which increases exponentially as the device is compressed. The patent specifically teaches a limitation of roll axis movement to 1/2 inch.
Another example of a standard side bearing is shown in U.S. Pat. No. 4,080,016. This bearing includes a pair of elastomeric blocks spaced apart by a saddle block. An upper surface of the saddle block has a cusp to separate a pair of roller bearings. Again, the patent teaches a mechanism to limit roll axis movement.
None of the prior art side bearings allows long-travel of the bearing to permit the necessary roll axis movement of the rail car body which has been found to be needed with an articulated rail car. This presents a problem in the use of certain articulated rail cars. In articulated rail cars, the extreme length of the rail car requires free torsional movement of the car body units, particularly as the units traverse a curved or banked portion of the rail. At such a curve, the outer rail is raised (or super-elevated) in relation to the inner rail to form a banked curve. In the approach to such a curve, there is a transition from the straight (not super-elevated) track to the curved (super-elevated) track. This causes a twisting of the rail car as it traverses this section of the track. The restriction of roll axis movement by the side bearings of the prior art prevents relative roll axis movement of one car unit with respect to another car unit. The restriction in roll axis movement can result in high twist loads being imposed at the articulation joint. This can apply such forces to the rail truck as to prevent the yaw axis rotation necessary for the car to successfully negotiate the curve.
The need for a long-travel side bearing has arisen as the units of articulated rail cars have become stiffer in torsion. This increase in stiffness is a by-product of the increasing load-carrying capacity of the units. The increased capacity units have exhibited a tendency to derail upon the entry to curves by forcing over the inner or low rail.
In such circumstances, as an articulated car enters a curve, the entire five unit car is twisted in the banking of the curve. On these cars, the connection of one unit to another is through an articulated connector. The connector has a female part that sits in the truck bowl in the same manner that the center plate of a standard freight car sits in the truck bowl. The male portion of the connector is fabricated into the adjacent unit. The connection is formed by the male portion fitting into the female portion. The male portion does not necessarily make contact with the truck bolster.
During the turning of the car on the banked rail curve, the short travel of a standard short-travel bearing prevents the rotation of the male portion within the female portion. The result is the placement of a very high load on the side bearing. This high side bearing load applied at a significant distance from the center of yaw rotation of the truck causes the application of a large yaw moment to the truck as it enters the curve. This yaw moment interferes with the proper yaw motion of the truck. Under certain conditions, particularly those where rail lubrication is present only on the upper or high rail, the result is excessive force on the low rail and the potential rolling over the low rail.